Type inference: bug or feature?

[Justin Dekeyser]

Hello,

Thank you for your input!
Indeed, the reason of such a choice sounds legit and confirms the idea of
having more safety.

I just came with this situation while playing around with Java, and I was
surprised;
But it definitely was an academic example that was not planned for real use
cases! ;-)

Thank you very much,

Have a nice day,

Justin Dekeyser



On Mon, Jul 27, 2020 at 12:55 PM Remi Forax <forax at univ-mlv.fr> wrote:

> ----- Mail original -----
> > De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> > À: "Justin Dekeyser" <justin.dekeyser at gmail.com>, "core-libs-dev" <
> core-libs-dev at openjdk.java.net>
> > Envoyé: Lundi 27 Juillet 2020 12:26:40
> > Objet: Re: Type inference: bug or feature?
>
> > CC'ing compiler-dev
> >
> > Hi Justin,
> > the behavior you are observing is normal. For a Java expression to be
> > able to be influenced by type inference it has to be a poly expression
> > (where poly stands for _many_ as in, the same expression can have many
> > types).
> >
> > At the time we did Java 8 we briefly considered making cast expressions
> > part of the poly expression dance (like lambdas, method references,
> > method calls, parens, new creation expression, conditionals and switch
> > expression - see JLS 15.2), but the rule which dictate cast conversion
> > (JLS 5.5) are so complex (as they have to take into account possibility
> > for unchecked cast, etc.) that it felt like touching them was a very
> > risky move, with no clear benefit.
>
> There was another reason :)
>
> The idea behind introducing generics is to get ride of casts because they
> are a hazard at runtime.
> So allowing people to use casts as poly expression goes in the wrong
> direction, add more casts.
>
> Instead of using a cast, it's better to use explicit type arguments,
>   TheClass.<TheTypeYouWantHere>emptyList()
>
> [...]
>
> >
> > Cheers
> > Maurizio
>
> regards,
> Rémi
>
> >
> > On 26/07/2020 18:22, Justin Dekeyser wrote:
> >> Dear all,
> >>
> >> I'm not sure but I think I've found a bug in Java type inference
> mechanism.
> >> It may also be a feature, so I'll expose the problem to you below; in
> terms
> >> of Integer, Number and List, although you'll quickly realize it will
> work
> >> wrong in any similar situation.
> >>
> >> Let's assume I have
> >>
> >> static <U, V extends U> List<U> emptyList(Class<U> magnet) {
> >>     return Collections.emptyList();
> >> }
> >>
> >> Then the following codes does not compile (for the same reason):
> >>
> >> var x = (List<Integer>) emptyList(Number.class);
> >> List<Integer> x = (List<Integer>) emptyList(Number.class);
> >>
> >> incompatible types: List<Number> cannot be converted to List<Integer>
> >>
> >> however, the following do compile:
> >>
> >> var x = emptyList(Number.class); // inferred to List<Number>
> >> List<Integer> x = emptyList(Number.class); // no mistake here, it's
> Integer
> >> on the left
> >>
> >> Is this the expected behavior? Why does casting operation here interfere
> >> with type inference like this?
> >>
> >> Regards,
> >>
> >> Justin Dekeyser
> >>
> >>
> >>
> >>
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